The lack of any change to the lineup is something of a surprise. The roster has been in flux since Simon Cowell left in 2010, and the show’s ratings bottomed out last season. “Idol” averaged 10.1 million total viewers, a 30 percent drop from the previous year and a far cry from the 30 million who would watch the show on average during its heyday, when it was the most popular program on television. A number of factors have contributed to the show’s decline, including its aging format and continued competition from NBC’s “The Voice.” Fox has previously said that it will cut back on the number of hours “Idol” will be on during the week next season. The premiere is scheduled for January.